The invention relates to an x-ray diagnostic installation utilizing a television pickup system, and comprising an image store, a difference stage for providing subtraction images through difference formation from the stored and the actual video signal, and a monitor. Subtraction images are employed in the case of fluoroscopic examination, particularly in the case of selective angiography, in order to make clearly visible blood vessels which, in the normal x-ray image, can only be poorly recognized or on which bone structures are superimposed.
In the case of fluoroscopic subtraction, images with contrast medium filling are taken and stored and subtracted from images without contrast medium filling. A subtraction of two picture records can proceed photographically or by television-technical means. In the case of television subtraction which is predominantly in use today, the image taken during the presence of a contrast medium is stored either by means of a photographic medium which can be scanned by a second television camera, or by means of a video image store. In an electronic difference amplifier the video signals are subtracted from one another. The resulting difference images are displayed on a monitor and predominantly contain only the representation of the blood vessels with the contrast medium present.
Since all vessels must be simultaneously filled for a representation, in the case of an image taken during the presence of the contrast medium, high contrast medium quantities must be employed. In order to take contrast medium images, the entire injection operation is recorded in a video store. The photograph (or radiograph) best representing the vascular tree is selected and employed for subtraction. The subtraction images thus prepared exhibit an image quality which is not always satisfactory. Moreover, many manipulations must take place, so that the method for preparing subtraction images is cumbersome. Furthermore, through this known method, the patient is greatly stressed due to high contrast medium quantities and a large x-ray dose.
In the German OS No. 2,919,425, it is therefore proposed, for the storage of the video signals, to previously conduct an integration over several images. These stored signals and the current video signals are subtracted. The difference signals are displayed on the monitor. Although the quantum noise is improved in this way, an optimum representation of the vessel paths with low contrast medium quantities does not occur.